1. Field of the Invention
This invention relates to structures and fabricating methods for microwave ferroelectric phase shifters.
One aspect of the invention relates to a fabrication technique wherein a ferroelectric phase shifter element is formed on an easy-to-handle slab of ferroelectric material, and the product thus obtained. A further aspect of the invention relates to an assembly comprising a plurality of ferroelectric phase shifter elements all formed on a common slab of ferroelectric material, which can thereby be commonly inserted into a plurality of phase shifter circuits.
The invention reduces fabrication costs, eases the assembly process, and produces a more uniform microwave ferroelectric phase shifter. This invention will find applications at all microwave frequencies, but is expected to have an impact especially at frequencies above 10 GHz, where current assembly methods are expensive and uniform phase shifter performance is difficult to achieve.
More particularly, the invention will reduce the difficulty in handling, metallizing, and positioning small, fragile pieces of ferroelectric material. By fabricating several phase shifters on a single piece of ferroelectric material, the multiple phase shifters thus obtained can be expected to find applications in electronic scanning antennas, where from several tens to several thousands of phase shifters are required in each antenna. This invention solves the problem of individually fabricating and assembling phase shifters, for microwave systems which require many phase shifters. This invention will reduce the cost when several phase shifters are required, and produce more uniform performance by eliminating assembly variations.
2. Background Art
Ferroelectric phase shifters are used to control the amount of phase shift of a microwave signal, by varying the permittivity of the ferroelectric material. The permittivity can be controlled by an applied electric field. A phase shifter of background interest is disclosed in U.S. Pat. No. 5,032,805. Because of the high dielectric constant of ferroelectric materials, these phase shifters are very small devices, and become increasingly smaller at higher frequencies. Ferroelectric phase shifter dimensions above 10 GHz are of the order of a few mils, one mil being equal to about 0.0254 mm, which makes them difficult to handle. Breakage is common when positioning the ferroelectric into the phase shifter circuit.
Previous microstrip ferroelectric phase shifters have used a ferroelectric rod as the active phase shifting element. FIG. 1 shows a known ferroelectric phase shifter circuit 12, which uses a rod 10 made of barium strontium titanate ferroelectric material having a dielectric constant of, for example, between 100 and 6000. The rod 10 is arranged in a hole 14 which is cut in the dielectric substrate 16 to enable the rod 10 to be positioned in the circuit 12. If the material has a nominal dielectric constant of 800, for example, the size of the rod required to produce 360 degrees of phase shift at 10 GHz is 0.008".times.0.010".times.0.45". It is difficult to position such a small rod consistently in the phase shifter circuit. Experience has shown that breakage is a common occurrence during the positioning process. For higher frequency applications, the task of handling the ferroelectric rods will be even more difficult; at 30 GHz the dimensions of the rod become 0.003".times.0.0035".times.0.15".
Other phase shifting circuits of interest are shown in U.S. Ser. No. 07/916,741 filed Jul. 22, 1992 (U.S. Pat. No. 5,212,463) and U.S. Pat. No. 4,105,959. The disclosures of these and all other prior art information mentioned herein is expressly incorporated by reference.
A known type of electronic scanning antenna, shown in FIG. 2, uses an individual ferroelectric phase shifter circuit 22a, 22b, etc., for each of a plurality of series radiating arrays 20a, 20b, etc. Each phase shifter circuit may have a DC voltage block 24, a pair of transition elements 26, and a bias voltage circuit 27, constructed and arranged in a known manner. Each phase shifter element such as a ferroelectric rod 28a, 28b, etc., must be individually positioned into the array. It would be significantly more cost-effective, and enhance performance if a multiple phase shifter element were used.
Current ferrite phase shifters cost several thousand dollars each, and require individual tuning to achieve uniform performance. Today's electronic scanning antennas use several hundreds or thousands of phase shifters, and even with lower-cost ferroelectric phase shifters now being developed, the individual handling and packaging of these will contribute to a higher cost than is desirable for many applications. The cost of ferroelectric phase shifters will be reduced by the proposed multiple phase shifters.